This invention regards an arrangement for establishing and maintaining tension in a riser of the type typically used for offshore petroleum production, where the riser constitutes a connection between the seabed and an installation on the surface of the sea.
When establishing a petroleum well offshore by means of a drilling vessel, it is normal to set in place a riser extending between the seabed and the drilling vessel, relatively early in the drilling phase. Besides being used for controlling drill tools, the riser is also used for carrying drill fluid between the well and the drilling vessel.
A riser is ordinarily formed as a length of tubing telescopically connected to the drilling vessel, where the lower portion of the riser is connected to seabed equipment. The dimensions and weight of the riser are considerable, and it is crucial to the function of the riser that tension be established and maintained in the riser, to ensure that the riser is subjected to insignificant compressive stresses only. If the riser is subjected to the load of its own mass, which may typically amount to several hundred tons, it will automatically break.
In order to establish and maintain sufficient tension in the riser, compensating means are normally connected between the drilling vessel and the upper portion of the riser. The compensating means typically comprise one or more hydraulic cylinders, where the pressure side of the cylinders are in communication with a hydraulic pump and hydraulic accumulators through associated pipes and a system of valves.
The heave motion of the drilling vessel and changes in the sea level are taken up by the telescope connection between the drilling vessel and the riser. The compensating means must be constructed so as maintain more or less constant tension in the riser, also during said motion, and the accumulators of the compensating means are therefore designed to receive and deliver the largest share of the pressure fluid that must be drained and supplied to the compensating cylinders during the motion of the drilling vessel.
It is obvious that hydraulic pumping and accumulator systems of the type referred to here are both very extensive and complicated, while also being costly both to procure, install and operate. Moreover, there is a danger that any interruption of power to the pumping system may after a while entail a risk of insufficient supply of pressure fluid to the hydraulic compensating cylinders, whereby the riser may be damaged.
The object of the invention is to remedy the disadvantages of prior art.
The object is achieved in accordance with the invention by the characteristics stated in the description below and in the appended claims.
In a heave compensating means according to the invention, the required pressure difference between the two fluids acting on the two piston sides of the compensating cylinders is effected by means of the hydrostatic pressure that exists under the surface of the sea.
In one embodiment in which the compensating cylinders are located by the surface of the sea, a bell in the form of a receptacle is arranged in the sea at a predetermined depth below the surface. The bell has an opening in its lower portion, where water may flow in and out. At its upper portion, the bell is connected in a communicating manner to the pressure side of the compensating cylinder by means of an interconnecting pipe/hose/conduit. Said interconnecting pipe is also connected to a compressor or other pressure source designed to supply the bell with air or another fluid having a significantly lower density than water.
By introducing such relatively light fluid to the pipe and the pressure bell, a fluid pressure is established, at the surface in the interconnecting pipe and the pressure sides of the compensating cylinders, which corresponds to the hydrostatic pressure difference in the water outside the bell and in the fluid in the bell.
In another embodiment, in which the telescopic section of the riser and the compensating cylinders are located at a considerable depth below the surface of the sea, the pressure sides of the cylinders are supplied with seawater through an opening in the cylinders, or with another fluid through an interconnecting pipe to the surface. The depressurised sides of the cylinders are connected to the surface via an interconnecting pipe that is under vacuum or filled with a relatively light fluid.
The operation of the arrangement is explained in greater detail in the specific part of the description with reference to the appended drawings.
An advancement of the invention may be to provide a separating tank/cylinder in the fluid supply for the cylinder, designed to supply the cylinder with a fluid that is different from the pressure fluid.